The problem I have with this is the idea that it's Meta's job to make sure kids do not see this content on FB or IG.
It's nonsense! Like you say, it's not like kids only saw their first boobs or butt at 18 even 20-25 years ago (so pre social media era).
And if they did somehow get their hands on say a nudie mag when they were hitting puberty, it's not like people back then blamed Playboy for their kids being confronted with nude bodies. People knew that it was the parents job to make sure their kids were not confronted with anything that the parents did not approve of.
For whatever reason some people have now convinced themselves that it's not their job but the job of the government and private companies to do this. That is bullshit.